JP5683598B2 - Improved diesel oxidation catalyst - Google Patents

Description

  The invention relates to a catalyst for purifying exhaust gas from a diesel engine, in particular when a further exhaust gas purification unit such as a particle filter and / or a nitrogen oxide reduction catalyst is installed downstream of a large vehicle. The present invention relates to an oxidation catalyst particularly suitable for purifying exhaust gas from the vehicle.

Crude exhaust gas from diesel engines contains carbon monoxide CO, hydrocarbons HC and nitrogen oxides NO _X in addition to relatively high oxygen content up to 15% by volume. In addition, the crude exhaust gas includes the release of particles mainly consisting of soot and possibly organic agglomerates, resulting from partially incomplete combustion of fuel in the cylinder. The polluted gas carbon monoxide and hydrocarbons can be easily rendered non-polluting by oxidation over a suitable oxidation catalyst. To remove particulate emissions, a diesel particulate filter with or without a catalytically active coating is a suitable device. It is difficult to reduce nitrogen oxides to nitrogen (“NO _X removal from exhaust gas”) due to the high oxygen content. A known method is the selective catalytic reduction (SCR) of nitrogen oxides over a suitable catalyst known as SCR catalyst for short. This method is currently preferred for removing NO _x from diesel engine exhaust. In the SCR method, nitrogen oxides present in the exhaust gas are reduced by using a reducing agent introduced into the exhaust gas system from an external source. Preferably, ammonia or a compound that releases ammonia, such as urea or ammonium carbamate, is used as the reducing agent. In some cases, ammonia generated in situ from the precursor compound reacts with nitrogen oxides in the exhaust gas by a leveling reaction on the SCR catalyst to form nitrogen and water.

  Combining various exhaust gas purification units is indispensable in order to comply with legal exhaust gas limits that will apply in Europe, North America and Japan in the future for diesel vehicles. Such exhaust gas purification systems have already been proposed and are currently being tested or prepared for mass production for many vehicle types (passenger vehicles and large vehicles).

Thus, EP 1054722 describes a system for treating diesel exhaust containing NO _x and particles, in which an oxidation catalyst is installed in front of the particle filter. Has been. A reducing agent source and a metering device for the reducing agent and the SCR catalyst are located downstream of the particle filter. US Pat. No. 6,928,806 also describes a system for removing nitrogen oxides and particles from diesel engine exhaust. Here, the SCR catalyst is first installed downstream of the oxidation catalyst together with the introduction of the reducing agent described above. A diesel particle filter is placed downstream of the SCR catalyst.

In such a combined system, the upstream oxidation catalyst must meet special requirements. The reaction that takes place on the oxidation catalyst is intended to treat the exhaust gas so that a very optimal exhaust gas purification result can be obtained also in the downstream unit. For example, it must be taken into account that when the optimal NO / NO ₂ ratio prevails at the inlet to the SCR catalyst, the SCR catalyst exhibits the best degree of conversion of nitrogen oxides. This optimal NO / NO ₂ ratio is about 1 for all currently known SCR catalysts. When the NO _X present in the exhaust gas is composed only of NO and NO _2, the optimum _NO 2 / NO _X ratio in the range from 0.3 to 0.7, preferably from 0.4 0.6 Up to a range of 0.5, particularly preferably 0.5. Whether this ratio is achieved in a system such as that described in US Pat. No. 6,928,806 is related to exhaust gas temperature, and therefore engine operating conditions and oxidation catalyst activity. Determined by. In a system such as that described in EP 1054722, another influential parameter is the diesel particulate filter configuration and soot load placed downstream of the oxidation catalyst. This is because the oxidation of soot by NO ₂ mainly forms NO in addition to CO and CO ₂ .

Another important requirement that the oxidation catalyst must meet in a combined system arises from the requirement that the downstream diesel particulate filter must be regenerated from time to time. In most systems, to adhere to the filter over a particle operating time, they, in oxygen or situ i.e. driven by NO _2, it is impossible to burn to the same extent as has been introduced into the filter. As a result, the reverse pressure of the exhaust gas increases in the filter. When the predetermined threshold is reached, regeneration of the active particle filter is triggered. That is, the particulate filter has a higher temperature level to exceed the soot ignition temperature, which may have been reduced by catalysis, and to burn the soot adhering to the filter with oxygen to form CO _2. Until heated. There are various strategies for heating the particle filter at the start of the regeneration phase. Established strategies include fuel injection into the cylinder combustion chamber during the piston exhaust stroke, or secondary injection into the exhaust gas system. The injected fuel is catalytically combusted on the oxidation catalyst, and the released reaction heat is introduced into the exhaust gas and used to heat the downstream particle filter to a temperature equal to or higher than the soot ignition temperature.

Thus, in a combined system with a particle filter and an SCR catalyst, the upstream oxidation catalyst provides long-term high thermal stability, good toxicity to sulfur-containing compounds (especially SO _x ), and an ideal low ignition temperature ( Not only the usual requirements such as very high CO and HC conversion at the light-off temperature) but also the following two additional requirements must be met. That is,
1. The degree of oxidation of NO must be well matched to the downstream SCR catalyst. That is, the NO ₂ / NO _X ratio produced on the oxidation catalyst should ideally be about 0.5 or above.
2. The oxidation catalyst must be well adapted as a “heating catalyst” for the downstream particle filter. That is, it must be possible to oxidize very large amounts of unburned hydrocarbons in a short time without stopping the oxidation reaction. The conversion of unburned hydrocarbons should be as complete as possible. This is because breakthrough of unburned hydrocarbons to the SCR catalyst placed further downstream through the oxidation catalyst may inhibit the catalytic action of the SCR catalyst. In addition, break-through of unburned hydrocarbons at the end of the exhaust gas system can lead to situations where legal limits are not observed. The combustion of fuel on the oxidation catalyst should ideally “ignite” at low exhaust gas temperatures (180 ° C. to 250 ° C.).
In general, the oxidation catalyst also exhibits very high HC conversion, even at very low exhaust gas temperatures, and HC conversion should ideally increase to a maximum value very quickly after reaching the light-off temperature. is there. Furthermore, the catalyst must have aging stability so that its activity is not excessively impaired by the heat of reaction released during the exothermic combustion of hydrocarbons. Hereinafter, these performance requirements are summarized and referred to as “heat-up performance”.

  Furthermore, the use of the catalyst in a combined system for purifying exhaust gas from diesel vehicles, for example, large commercial vehicles such as route buses, waste disposal vehicles, construction machinery or agricultural machinery, often with diesel passenger vehicles Must be considered to be used under basically different operating conditions. As a result, the characteristics of the different exhaust gases are such that the exhaust gas temperature is quite low and the composition of the exhaust gas is different. Thus, although the nitrogen oxide content in the crude exhaust gas is significantly less compared to the exhaust gas of a diesel passenger car, the rate of particle emission may increase considerably. The performance of the upstream oxidation catalyst must match the characteristics of such exhaust gases.

  Conventional oxidation catalysts cannot meet the above requirements, particularly those used in combination systems for purifying exhaust gases from heavy commercial vehicles.

Thus, for example, European Patent No. 0800856 by the present applicant (Patent Document 3) describes one or more zeolites present in the form of Na ⁺ or H ⁺ and further aluminum silicate (silicon dioxide / oxide). A diesel oxidation catalyst comprising a weight ratio of aluminum = 0.005 to 1), aluminum oxide, and titanium oxide, and one or more metal oxides selected from at least one platinum group metal is described. doing. Catalysts in which platinum group metals are deposited only on additional metal oxides can oxidize refractory long-chain paraffins in the exhaust gas, especially at temperatures below 200 ° C. However, this reaction is too slow at low temperatures and does not complete well, so breakthrough of unburned hydrocarbons occurs when the catalyst is used as a heated catalyst for active regeneration of the downstream filter. This catalyst is also not suitable for use in a combined system with an SCR catalyst because of its insufficient NO oxidation activity.

EP 1370357 by the present applicant describes a catalyst composed of a catalytically active coating on an inert honeycomb body composed of ceramic or metal. The coating contains at least one of platinum group metals platinum, palladium, rhodium and iridium on an ultrafine oxide support material mainly composed of silicon dioxide and having a large porosity. The support material comprises aggregates of substantially spherical primary particles having an average particle size in the range of 7 nm to 60 nm. This catalyst shows a good heat aging resistance and a tendency to decrease that its action is weakened by the sulfur-containing exhaust gas component (especially SO _x ). However, this catalyst does not show sufficient NO oxidation activity for a combined system with an SCR catalyst and does not show satisfactory heat-up performance.

  As mentioned above, most diesel oxidation catalysts contain only a uniformly structured functional coating. For example, a catalyst having two functional coatings of different compositions, such as known as a three-way catalyst for purifying exhaust gas from a spark ignition engine, tends to be rare in the case of diesel oxidation catalysts. US Patent Application Publication No. 2008/0045045 (Patent Document 5) describes such an oxidation catalyst for diesel. Among these, a bottom layer (“bottom washcoat layer”) or downstream zone (“downstream washcoat layer”) containing a high surface area support material comprising platinum and / or palladium and substantially free of silicon dioxide, It is applied to a support substrate. This bottom layer or downstream zone does not contain any HC storage portion (eg zeolite). A top layer (“top washcoat layer”) or upstream zone (“upstream washcoat layer”) containing a high surface area support material, platinum and / or palladium, and also an HC storage material is the bottom layer or downstream zone. It is applied upstream. The weight ratio of Pt: Pd in the top layer (or upstream zone) of the catalyst is greater than the weight ratio of Pt: Pd in the bottom layer (or downstream zone). The top layer (upstream zone) of the catalyst is optimized for sulfur resistance and paraffin oxidation and the bottom layer (downstream zone) is optimized for hydrothermal stability. However, this catalyst with a coating layer that is optimized with respect to sulfur resistance and is therefore acidic does not exhibit sufficient NO oxidation activity.

European Patent No. 1054722 US Pat. No. 6,928,806 European Patent No. 0800856 EP 1370357 US Patent Application Publication No. 2008/0045045

  It is an object of the present invention to purify diesel engine exhaust gas, particularly heavy commercial vehicle exhaust gas, suitable for use in a combined system that includes a particulate filter and an SCR catalyst along with reducing agent injection. The oxidation catalyst meets the above requirements better than the oxidation catalysts known in the prior art.

  This object is achieved by a catalyst for purifying diesel engine exhaust gas comprising a support and two catalytically active coatings having different compositions and only one of which is in direct contact with the exhaust gas flowing out. Both coatings contain the platinum group metals platinum (Pt) and palladium (Pd) as catalytically active components, and the coating that is in direct contact with the effluent exhaust gas contains more Pt than Pd. The catalyst is characterized in that the coating in direct contact with the effluent exhaust gas contains a greater amount of all platinum group metals than the coating not in direct contact with the effluent exhaust gas.

The coating present in the catalyst of the present invention performs different functions. Thus, the coating that is in direct contact with the flowing exhaust gas has an excellent oxidizing activity for HC and CO components, especially for NO components. A combination of a layer with an increased total precious metal content and a high Pt: Pd weight ratio reduces the oxidizing power of the coating that is in direct contact with the exhaust gas flowing out, and the coating with a low total precious metal content and not in direct contact with the exhaust gas exhausting. It is significantly larger than the oxidizing power of. As a result, the catalyst of the present invention shows virtually complete conversion of CO and HC, and has an excellent NO ₂ formation rate during the “normal operation stage”, irrespective of the active regeneration of the downstream particle filter. Show. With respect to the particulate filter and SCR catalyst of the exhaust gas purification component placed downstream of the combined system, this has two advantages. That is, as a result of an increase in the NO ₂ content of the exhaust gas, particles adhering to the filter that are oxidized and can be burned out in situ (ie, without additional heating means during normal operation). The rate increases. Thus, the formation of soot particle “filter cake” in the filter and thus the increase in the back pressure of the exhaust gas on the filter is reduced. The filter is less frequently played back. Furthermore, the excellent degree of formation of NO ₂ on the catalyst of the present invention during normal operation has a NO ₂ / NO _X ratio at the inlet to the downstream SCR catalyst in the range of 0.3 to 0.7. Guarantee that. As a result, an excellent degree of conversion of NO _x on the SCR catalyst is possible even at low temperatures (180 ° C. to 250 ° C.).

The coating that is in direct contact with the effluent exhaust gas preferably contains 1.2 to 2 times as much platinum group metal as the coating that is not in direct contact with the effluent exhaust gas. In a preferred embodiment of the catalyst of the present invention, 55% to 80% by weight, particularly preferably 55% to 70% by weight, ideally 57% to 60% by weight of the total noble metal present in the catalyst. In the coating, which is in direct contact with the effluent exhaust gas. Furthermore, the Pt: Pd weight ratio of the coating is preferably equal to or greater than 6: 1. The weight ratio of Pt: Pd is particularly preferably in the range 6: 1 to 20: 1, very particularly preferably in the range 6: 1 to 10: 1, ideally 7: 1. At that time, oxidizing power of the coating which directly contacts the exhaust gas flowing out, without unduly increasing the total amount of the noble metal must be used, without much increasing the amount of platinum particularly expensive noble metal, the NO ₂ It matches the required degree of formation very well.

  The second coating that is not in direct contact with the effluent exhaust gas contains a lower amount of total precious metal and has a significantly lower Pt: Pd weight ratio, i.e., a significantly higher relative amount of palladium. The Pt: Pd weight ratio of this coating which is not in direct contact with the effluent exhaust gas is preferably 1: 4 to 2: 1, particularly preferably 1: 2 to 1: 1. It exhibits a “heat up” function during active regeneration of the downstream particle filter and exhibits very good heat up performance (as described above).

Thus, in order to generally support HC conversion, particularly heat-up performance, a particularly preferred embodiment of the catalyst of the present invention is a beta-type zeolite, an X-type zeolite, a Y-type zeolite in a coating that is not in direct contact with the effluent exhaust gas. , Mordenite, and one or more zeolite compounds selected from the group consisting of ZSM-5 zeolite. These zeolites have a storage effect on the hydrocarbons produced in diesel exhaust. The presence of HC occluded zeolite in the coating that is not in direct contact with the effluent exhaust gas is incorporated into the zeolite, for example, during the cold start stage or as a result of its amount during the "heat up" stage for active filter regeneration. The hydrocarbons released again from the HC occlusion at a later point in time under suitable operating conditions must come into direct contact with the exhaust gas flowing out by forced flow and pass through a coating containing a large amount of noble metal. Has the advantage of not. This ensures that these hydrocarbons are converted to CO ₂ and water as completely as possible. This is because the coating in contact with the exhaust gas flowing out is a coating having a greater oxidizing power as described above. As a result, the catalyst of the present invention has better HC conversion performance over the entire driving cycle than in prior art catalysts where no HC storage zeolite compound is present and the catalyst is arranged in contact with the exhaust gas from which it exits. This is achieved in any suitable embodiment.

  According to the inventor's discovery, the combination of the two layers is the first time that an oxidation catalyst can meet all requirements that would make it compatible with a combined system at all economically viable noble metal contents. It makes it possible to provide a catalyst. The two technical effects of NO oxidation and heat-up performance that occur in different operating conditions of the catalyst are, according to the inventor's discovery, that when the three-dimensional array of coatings is maintained, It can only be sufficiently effective when it is in direct contact with the exhaust gas that contains and contains a large amount of platinum and has a greater oxidizing power.

  In a preferred embodiment of the catalyst of the invention, platinum and / or palladium is selected from the group consisting of aluminum oxide or aluminum silicon mixed oxide doped with aluminum oxide, zirconium oxide and / or titanium oxide in two layers. Applied to one or more high melting point, high surface area supporting oxides. The selected support oxide is suspended in water to produce a suitable coating suspension. In the form of a suitable water-soluble precursor compound such as palladium nitrate or hexahydroxyplatinic acid, while platinum and palladium are agitated and fixed to the support material by setting the pH and / or adding auxiliary reagents as necessary To be added to the suspension. Suitable precursor compounds and auxiliary reagents are known to those skilled in the art. The suspension thus obtained is then agitated and added to the inert support by one of the conventional coating methods. After each coating step, the coated part is dried with a stream of hot air and baked if necessary. As a support for the catalytically active coating, it is advantageous to use ceramic or metal vented honeycomb bodies for producing the catalyst of the invention.

  There are various possible configurations for the coating on the support. FIG. 1 shows a preferred embodiment.

  A catalytically active coating (2) that is not in direct contact with the effluent exhaust gas is preferably applied directly to the vented honeycomb body and extends over the entire length of the component, as shown in FIG. The side is covered over the entire length of the component by a coating (1) that is in direct contact with the exhaust gas that flows out. This forms a two-layer catalyst or “layer catalyst”, which is rich in palladium, optionally with a “heat-up” functional coating containing zeolite as a bottom layer and rich in platinum. Yes, it is completely covered as a top layer by a coating with high oxidizing power.

Furthermore, the catalytically active coating (2) that is not in direct contact with the exhaust gas flowing out in the catalyst of the present invention extends only 5% to 50% of the length of the component on the inflow side to form an upstream zone It can be applied to the ventilation honeycomb body. The coating (1), which is in direct contact with the effluent exhaust gas, then extends over the remaining length of the component to form an adjacent downstream zone. FIG. 1 c) shows such an embodiment as a “zone catalyst”. As an advantage of the zone catalyst, the zone length can be easily matched to the performance characteristics required by the combined system in which the catalyst is used. When the particle filter is first provided as the catalyst of the present invention, and then the SCR catalyst, the NO ₂ / NO _X ratio (0.6 to 0.9) is relatively high as a result of burning out soot by NO ₂ during normal operation. Will be provided. The downstream zone can cover 70% to 95% of the length L of the ventilation honeycomb body. For example, the upstream “heat-up” may occur when frequent active regeneration of the downstream particle filter is required due to the special operating characteristics of commercial vehicles where a very large amount of particles is produced with relatively cool exhaust gases. The length of the coating zone can easily be 40% to 50% of the length of the ventilation honeycomb body.

  Regardless of whether the catalyst of the present invention is configured as a “layer catalyst” or “zone catalyst”, the (bottom) layer or upstream zone applied directly to the vented honeycomb body is also a beta zeolite, X type It is particularly advantageous to contain one or more zeolite compounds selected from the group consisting of zeolites, Y-type zeolites, mordenites, and ZSM-5 zeolites and exhibit occlusion with respect to hydrocarbons produced in diesel exhaust gas. is there. As mentioned above, the addition of this zeolite generally results in improved HC conversion, particularly “heat up performance”.

The catalyst of the present invention is suitable for use in an apparatus for purifying exhaust gas from a diesel engine. Such a device particularly preferably further comprises a catalyst for the selective catalytic reduction of diesel particulate filters and / or nitrogen oxides, the catalyst of the present invention being a diesel particulate filter and / or a selective catalyst for nitrogen oxides. Installed upstream of the catalyst for reduction.
For example, the present invention provides the following items.
(Item 1)
A catalyst for purifying exhaust gas of a diesel engine, comprising: a support (3); and two catalytically active coatings having different compositions, wherein one (1) is in direct contact with the exhaust gas flowing out. Both coatings contain the platinum group metals platinum (Pt) and palladium (Pd) as catalytically active components, and the coating (1) in direct contact with the effluent exhaust gas is more Pt than Pd. In a catalyst containing a lot,
The coating (1) in direct contact with the outflowing exhaust gas contains a greater amount of total platinum group metal than the coating (2) not in direct contact with the outflowing exhaust gas, catalyst.
(Item 2)
The coating (1) in direct contact with the outflowing exhaust gas contains 1.2 to 2 times as much platinum group metal as the coating (2) not in direct contact with the outflowing exhaust gas. The catalyst according to item 1.
(Item 3)
Catalyst according to item 2, characterized in that the Pt: Pd weight ratio of the coating (1) in direct contact with the effluent exhaust gas is equal to or greater than 6: 1.
(Item 4)
4. The catalyst according to item 2 or 3, characterized in that the coating (2) that does not directly contact the flowing exhaust gas has a Pt: Pd weight ratio of 1: 4 to 2: 1.
(Item 5)
One or more high melting point, high surface area supports wherein platinum and / or palladium are selected from the group consisting of aluminum oxide, aluminum oxide doped with zirconium oxide and / or titanium oxide, and aluminum-silicon mixed oxides 5. Catalyst according to any one of items 1 to 4, characterized in that it is applied to both layers on the oxide.
(Item 6)
6. Catalyst according to any one of items 1 to 5, characterized in that both catalytically active coatings are applied to a ceramic or metal vented honeycomb body as support (3).
(Item 7)
The catalytically active coating (2) that is not in direct contact with the effluent exhaust gas is applied directly to the vented honeycomb body (3) and extends over the entire length of the component and the effluent over the entire length of the component. 7. Catalyst according to item 6, characterized in that the exhaust gas side is covered by the coating (1) in direct contact with the exhaust gas that does.
(Item 8)
The catalytically active coating (2) that is not in direct contact with the outflowing exhaust gas is applied to the vented honeycomb body (3) and extends upstream to the upstream side over only 5% to 50% of the length of the component. Item 6. characterized in that the coating (1) forming a zone and in direct contact with the outgoing exhaust gas extends over the remaining length of the component to form an adjacent downstream zone. catalyst.
(Item 9)
The layer (2) applied directly to the vented honeycomb body or the upstream zone (2) is selected from the group consisting of beta zeolite, X zeolite, Y zeolite, mordenite and ZSM-5 zeolite 9. The catalyst according to item 7 or 8, further comprising a zeolitic compound or higher and exhibiting an occlusion action with respect to hydrocarbons generated in the diesel exhaust gas.
(Item 10)
An apparatus for purifying the exhaust gas of a diesel engine, comprising the catalyst according to any one of items 1 to 9.
(Item 11)
Item 10. The item according to item 10, characterized in that the catalyst according to any one of items 1 to 9 is placed upstream of the catalyst for the selective catalytic reduction of diesel particulate filters and / or nitrogen oxides. Equipment.

The invention is illustrated below by means of several drawings and examples.

Various embodiments of the catalyst of the present invention are shown, and the arrows indicate the flow direction of the exhaust gas to be purified. FIG. 1a) is an overall schematic view showing a vented honeycomb body (3) having a length L and containing catalytically active coatings (1) and (2). FIG. 1 b) shows the configuration as a “layer catalyst” as a cross-section of a vented honeycomb body (3) showing just one flow path, with a catalytically active coating (2) that is not in direct contact with the exhaust gas that flows out. It is applied directly to the ventilation honeycomb body (3), and is covered with the coating (1) that is in direct contact with the exhaust gas flowing out over the entire length of the ventilation honeycomb body (3). FIG. 1c) shows the configuration as a “zone catalyst” as a cross-section of a vented honeycomb body (3) showing just one flow path, with a catalytically active coating (2) not in direct contact with the exhaust gas flowing out. The coating (1), which is configured as the upstream zone, covers 5% to 50% of the length of the vented honeycomb body and is in direct contact with the exhaust gas flowing out, forms the downstream zone and the remaining length of the honeycomb body Is covered. The NO ₂ yield of the catalyst K1 according to the invention is shown as a function of the temperature upstream of the catalyst compared to the single layer catalyst CK1 according to the prior art. The HC breakthrough ("HC slip") via the catalyst K1 of the present invention at the operating point where the HC throughput was significantly increased via the catalyst is shown in comparison with the prior art catalyst CK1. The NO ₂ yield in the coating CK3 in direct contact with the exhaust gas flowing out, in comparison with the NO ₂ yield in the coating CK2 catalyst K1 of the present invention which is not in direct contact with the exhaust gas flowing out, a function of the temperature upstream of the catalyst As shown. The catalyst K1 of the present invention that does not directly contact HC breakthrough ("HC slip") that has passed through the coating CK3 that is in direct contact with the flowing exhaust gas at the operating point where the amount of HC treated has increased significantly through the catalyst. It is shown in comparison with the HC breakthrough through the coating CK2. The NO ₂ yield of the catalyst K2 of the present invention that does not contain any zeolite compound in the coating that does not come into direct contact with the exhaust gas that flows out, Shown as a function of temperature upstream of the catalyst compared to NO ₂ yield. HC breakthrough ("HC slip") via the catalyst K2 of the present invention, which does not contain any zeolite compound in the coating that does not come into direct contact with the exhaust gas that flows out, is added to the coating that does not come in direct contact with the exhaust gas that flows out. It is shown in comparison with the HC breakthrough through the catalyst K3 of the present invention. The NO ₂ yield of the comparative catalyst CK5 with a three-dimensional array of functional layers according to the invention and a zeolite compound in the coating that is not in direct contact with the effluent exhaust gas, with the same composition but with the reverse three-dimensional array of layers It is shown as a function of the temperature upstream of the catalyst compared to the NO ₂ yield of some comparative catalyst CK4.

A catalyst according to the present invention and several comparative catalysts were prepared. For this purpose, a ceramic honeycomb body of 62 cells / cm ^{2 having} a diameter of 266.7 mm, a length of 152.4 mm and a cell wall thickness of 0.1651 mm was coated with a coating suspension having the composition described later by a conventional dipping method. Coated. After application of the coating suspension, the honeycomb body was dried with a hot air blower and heat-treated at 500 ° C.

  The catalytic activity of the finished catalyst was tested on an engine test bench (Euro-IV) equipped with a MAND2066 common rail diesel engine with a solid volume of 10.5 liters. The test stand had a temperature measurement point upstream of the catalyst and detailed exhaust gas analysis equipment upstream and downstream of the catalyst.

Prior to testing, the catalyst was first subjected to artificial aging. For this purpose, the catalyst was occluded in a furnace for 16 hours in a hot water atmosphere (10% by volume H ₂ O and 10% by volume O _{2 in} air) at a temperature of 750 ° C.

In order to test the NO oxidation performance of the catalyst, a “light-off test” was performed. Here, the catalyst is subjected to the following specified conditions:
Rotational speed: 1100 / min Torque characteristics: 0 → 2130 Nm (Time = 1800 seconds)
2130 Nm (Time = 30 seconds)
2130-> 0 Nm (at time = 1800 seconds)
→ Temperature upstream of catalyst: 115-455 ° C
Below, heat in the exhaust gas to be purified.
During this time, the concentrations of NO and NO ₂ in the exhaust gas upstream and downstream of the catalyst were measured with a chemiluminescence detector (CLD; AVL) at a frequency of 1 Hz. From these data, the NO ₂ yield as a function of temperature could then be determined by the following equation:

here,

It is.

  In order to test “heat-up performance”, the following operating points were set continuously.

  The fraction of residual hydrocarbons that breakthrough the oxidation catalyst was determined in ppm by volume with a measurement frequency of 1 Hz by a flame ionization detector (FID, AVL).

Comparative Example 1
A conventional diesel oxidation catalyst with only one active layer was produced. In order to produce a suitable coating suspension, a silicon-aluminum mixed oxide containing up to 20% by weight of SiO ₂ was impregnated with platinum nitrate solution and palladium nitrate solution to fill the pores and dried. . After heat setting of the noble metal, the powder thus obtained was suspended in water and added to the ceramic honeycomb body as described above. Based on the volume of the honeycomb body, the completed catalyst CK1 after drying and firing was included:
100 g / l silicon-aluminum mixed oxide containing up to 20% by weight of SiO ₂ 0.681 g / l platinum from nitrate solution 0.272 g / l palladium from nitrate solution

Example 1
A catalyst according to the present invention was prepared in which the total precious metal content and the platinum-palladium ratio corresponded to those of a conventional diesel oxidation catalyst as in Comparative Example 1.

For this purpose, a ceramic honeycomb body was first provided with a first coating having the following composition based on the volume of the finished catalyst:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.204 g / l platinum from nitrate solution 0.204 g / l palladium from nitrate solution 15 g / l commercial beta type Zeolite

  This coating became a coating that was not in direct contact with the exhaust gases that flowed out after production of the finished catalyst.

After drying and calcining the first layer, a second layer having the following composition was applied based on the volume of the finished catalyst:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.477 g / l platinum from nitrate solution 0.068 g / l palladium from nitrate solution

  This coating becomes a coating that comes into direct contact with the exhaust gas that flows out after production of the finished catalyst. Its composition is based on the finished catalyst K1 after drying and calcination.

FIG. 2 shows the NO ₂ yield of the catalyst K1 according to the invention as a function of the temperature upstream of the catalyst compared to the single-layer catalyst CK1 according to the prior art. The yield of NO ₂ obtained using the catalyst according to the invention is as high as 20% in the same temperature range.

  FIG. 3 shows the HC breakthrough observed downstream of the catalyst at the above operating point of the “heat up test” for the catalyst K1 according to the invention and the catalyst CK1 according to the prior art. The catalyst according to the invention exhibits a significantly lower HC breakthrough than the prior art catalyst CK1 at 6 of the 8 operating points tested.

  Utilizing two comparative catalysts CK2, CK3, the functionality of the two coatings present in the catalyst according to the invention was investigated separately from each other.

Comparative catalyst 2
The ceramic honeycomb body was provided with a coating having the following composition based on the volume of the finished catalyst CK2:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.204 g / l platinum from nitrate solution 0.204 g / l palladium from nitrate solution 15 g / l commercial beta type Zeolite

  This coating corresponds to the bottom coating of the catalyst K1 of Example 1 which is not in direct contact with the exhaust gas flowing out.

Comparative catalyst 3
The ceramic honeycomb body was provided with a coating having the following composition based on the volume of the finished catalyst CK3:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.477 g / l platinum from nitrate solution 0.068 g / l palladium from nitrate solution

  This coating corresponds to the top coating of catalyst K1 of Example 1 in direct contact with the effluent exhaust gas.

FIG. 4 shows the yield of NO ₂ in the catalysts CK2 and CK3. As is apparent, the catalyst CK3 corresponding to the coating in direct contact with the exhaust gas flowing out in the catalyst K1 according to the present invention has a significantly higher NO ₂ yield than the catalyst CK2 corresponding to the coating not in direct contact with the exhaust gas flowing out. I will provide a.

  FIG. 5 shows HC breakthrough with catalysts CK2 and CK3 which are equivalent for both layers. More interesting when compared to the HC breakthrough of the catalyst K1 of the present invention in FIG. 2: As shown in FIG. 3, the present invention is combined with two coatings that each do not exhibit particularly good “heat-up performance” In the case of forming a catalyst according to HC, the HC breakthrough in the catalyst thus obtained is dramatically reduced in the case of a two-layer three-dimensional arrangement according to the invention. At operating points 6, 7 and 8 where very high HC throughput occurs, the HC breakthrough ranges from 3000 to 3500 ppm by volume specific to the individual layers, less than 1000 ppm by volume at operating points 6 and 8, and 7 to less than 1500 ppm by volume. This effect is not to be expected as such in terms of the individual performance of each functional coating. The reason is the synergistic interaction between the coating that contains a large amount of palladium and does not come into direct contact with the exhaust gas that flows out, and the second coating that comes into direct contact with the exhaust gas that flows out. Outflowing exhaust gas is passed through a coating with higher oxidizing power. As a result, residual hydrocarbons that cannot be converted by "heat-up coating" alone or have been desorbed too quickly are oxidized. This provides a significant reduction in HC breakthrough with the catalyst of the present invention, and thus excellent “heat up performance”.

  The two catalysts K2, K3 according to the invention clearly show the influence of the additional zeolite in a coating that is not in direct contact with the exhaust gas that flows out.

Example 2
Using a procedure similar to that described in Example 1, an inventive catalyst K2 having two superimposed layers and having the following composition was prepared:
First layer = bottom layer = coating not in direct contact with outflowing exhaust gas:
92 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.302 g / l platinum from nitrate solution 0.302 g / l palladium from nitrate solution Second layer = top layer = outflow Coating in direct contact with exhaust gas:
45 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.706 g / l platinum from nitrate solution 0.101 g / l palladium from nitrate solution

Example 3
Using a procedure similar to that described in Example 1, an inventive catalyst K2 having two superimposed layers and having the following composition was prepared:
First layer = bottom layer = coating not in direct contact with outflowing exhaust gas:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.303 g / l platinum from nitrate solution 0.303 g / l palladium from nitrate solution 15 g / l commercial beta Zeolite second layer = top layer = coating in direct contact with effluent exhaust gas:
45 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.706 g / l platinum from nitrate solution 0.101 g / l palladium from nitrate solution

As shown in FIG. 6, the addition of zeolite in the coating that is not in direct contact with the effluent exhaust gas has no significant effect on the NO ₂ yield of the catalyst.

  However, as shown in FIG. 7, such addition of zeolite significantly reduces the HC breakthrough by the catalyst and thus has a very positive impact on the “heat up performance” of the catalyst.

  Furthermore, it was investigated what fundamental effect the three-dimensional arrangement of functional layers has on catalyst performance. For this purpose, two other comparative catalysts were produced:

Comparative Example 4
A procedure similar to that described in Example 1 was used to produce a two-layer comparative catalyst CK4 having the following composition based on the volume of the finished catalyst:
First layer = bottom layer = coating not in direct contact with outflowing exhaust gas:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.681 g / l platinum from nitrate solution 0.068 g / l palladium from nitrate solution Second layer = top layer = outflow Coating in direct contact with exhaust gas:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.204 g / l palladium from nitrate solution 15 g / l commercial beta zeolite

Comparative Example 5
A procedure similar to that described in Example 1 was used to produce a two layer comparative catalyst CK5 having the following composition based on the volume of the finished catalyst:
First layer = bottom layer = coating not in direct contact with outflowing exhaust gas:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.204 g / l palladium from nitrate solution 15 g / l commercial beta zeolite Second layer = top layer = outflowing exhaust gas Coating in direct contact with:
40 g / l silicon-aluminum mixed oxide containing up to 20 wt% SiO ₂ 0.681 g / l platinum from nitrate solution 0.068 g / l palladium from nitrate solution

FIG. 8 shows the yield of NO ₂ in the catalysts CK4 and CK5. As clearly shown in FIG. 8, the construction of coatings that do not contain zeolite with greater oxidizing power, such as coatings that do not come into contact with the effluent exhaust gas, have a significant adverse effect on the NO oxidizing power of the catalyst.

Claims (8)

Catalyst for purifying exhaust gas of a diesel engine, two catalytically active coatings having a different composition from the support (3) and in direct contact with the exhaust gas from which only one coating (1) flows out Both coatings contain the platinum group metals platinum (Pt) and palladium (Pd) as catalytically active components, and the coating (1) in direct contact with the effluent exhaust gas is more Pt than Pd. In the catalyst containing more
Wherein the coating in direct contact with the exhaust gas flowing out (1) than said outlet for exhausting gas in direct contact with no co Computing (2), containing more total platinum group metal weight in weight, where, Catalyst characterized in that the Pt: Pd weight ratio of the coating (1) in direct contact with the effluent exhaust gas is equal to or greater than 6: 1.   The catalyst according to claim 1, characterized in that the Pt: Pd weight ratio of the coating (2) not in direct contact with the flowing exhaust gas is 1: 4 to 2: 1. Platinum and / or palladium is applied to both coatings on the support, wherein aluminum oxide, aluminum oxide doped with zirconium oxide and / or titanium oxide, and aluminum-silicon mixed oxides. In a suspension composed of one or more selected high melting point, high surface area supporting oxides, the platinum and / or palladium is replaced with a suitable water soluble precursor compound prior to coating the support. Catalyst according to any one of claims 1 or 2, characterized in that it is added in the form.   Catalyst according to any one of the preceding claims, characterized in that both catalytically active coatings are applied to a ceramic or metal vented honeycomb body as support (3).   The catalytically active coating (2) that is not in direct contact with the exhaust gas flowing out is applied directly to the vented honeycomb body (3), extends over the entire length of the vented honeycomb body (3), and the vented honeycomb body (3 The catalyst according to claim 4, characterized in that the exhaust gas side is covered by the coating (1) in direct contact with the effluent exhaust gas over the entire length of).   One or more zeolites wherein the catalytically active coating (2) applied directly to the vented honeycomb body is selected from the group consisting of beta zeolite, X zeolite, Y zeolite, mordenite and ZSM-5 zeolite The catalyst according to claim 5, further comprising a compound and exhibiting an occlusion action with respect to hydrocarbons generated in the diesel exhaust gas.   An apparatus for purifying the exhaust gas of a diesel engine, comprising the catalyst according to any one of claims 1 to 6. Claim 7, wherein the catalyst according to any one of claims 1 to 6, characterized in that it is placed upstream of the catalyst for the selected catalytic reduction of the diesel particulate filter and / or nitrogen oxides The device described in 1.